Image projection systems such as light valve projection systems use one or more small flat panel electro-optic display devices to generate an image by modulating a light beam generated by a lamp This image is then magnified and projected on a screen for viewing.
Typically, the flat panel display device is an active matrix liquid crystal display (AMLCD) device, having, for example, a resolution of 1280 by 1024 pixels. Since the image is projected, the display device may be relatively small, e.g., less than about 6 cm. Further, the preferred mode of operation is a reflective mode, which allows use of thinner layers of liquid crystal light modulation material and correspondingly faster response times, since the light passes through the liquid crystal twice.
In conventional projectors using white light illumination, the light is split or filtered into three primary color components, red, green and blue, representing "corner" of a color triangle defining a color gamut on the CIE 1931 color chart, so that a complete rendering of the colors within the triangle can be achieved. These three components are separately modulated.
Known LCD projection systems provide one or more LCD panels, up to, for example, six panels, to modulate the spatial and color aspects of an image. Thus, in a single LCD panel display, the LCD is time or space division multiplexed with the respective colors. In a time-multiplexed system, red, green and blue polarized light is sequentially projected onto the panel, spatially modulated, analyzed (by, e.g., a second polarizer) and then projected, with the human eye merging the colors. In a spatially multiplexed system, a tri-color masked LCD panel is illuminated with white polarized light, with the modulated light passing through an analyzer (a second polarizer) and then projected. These single panel systems are known to be inefficient. For example, in a time or space domain multiplexed system, the peak brightness is much less than one-third of the brightness of the polarized white light source, for example utilizing as little as only 1% of the light output for typical images.
Image projection systems of high illumination efficiency allow the use of smaller projection lamps, smaller power supplies, reduced air flow cooling (resulting in reduced noise), greater portability, battery operation, and other advantages. In addition, the availability of higher power lamps may be limited, and thus higher efficiency may result in more available light sources.
In order to provide improved light use efficiency, four-color (red, green, blue, white) techniques were developed, in which the white light increases luminance by introducing white light into the image in proportion to the luminance signal. The amount of white light that may be acceptably added to the red, green, and blue light at an image point is a function of both the color and the luminance of that point. While there are many different algorithms to determine the amount of white to add, they share many common features. For saturated colors, very little white may be added to the red, green, blue. For desaturated colors, increasing amounts of white may be added.
The final luminance is L.sub.R +L.sub.G +L.sub.B +L.sub.W. Viewing tests have shown that if L.sub.W is greater than about 40-50% of L.sub.R +L.sub.G +L.sub.B, the colors are unacceptably desaturated and the primaries are unacceptably dim. This limits the overall gain in lumens possible with this technique.
It is also known to use colors other than red, green, blue for forming an image in a projection system. Alternatives include four or more spectral bands, and a six color system including red, green, blue, magenta, cyan and yellow. See, JP 09-230301 and U.S. patent application Ser. No. 08/579,655, expressly incorporated herein by reference.